Code Division Multiple Access (CDMA) is a multiple access method based on spread spectrum used in cellular communication systems. Other access techniques used in cellular communication systems include Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), and more recently, Orthogonal Division Multiple Access (OFDM). In CDMA, the narrow band data signal of a user is spread across a relatively wide frequency band using a spreading code having a broader bandwidth than the data signal. Typically, many users transmit simultaneously using that same frequency band. An individual spreading code is also used on each connection between the base station and the mobile station so that individual user signals may be distinguished from each other at a receiver based on the user's spreading code. Mutually orthogonal spreading codes are desirable because they do not correlate with each other.
Correlators or matched filters in CDMA receivers are synchronized with the desired signal identified by spreading code. The received data signal is returned in the receiver onto the original band by despreading it using the same spreading code as in the transmitter. The received data signals spread by some other spreading code do not correlate well and appear as noise from the point of view of the desired signal. The aim then is to detect the signal of the desired user among several interfering signals. In practice, the spreading codes are not completely non-correlated, and the signals of other users complicate the detection of the desired signal by distorting the received signal. This interference users cause each other is termed multiple access interference.
The mutual access interference caused by simultaneous users is a key factor affecting the capacity of a CDMA cellular communication system. The interference may be reduced by attempting to keep the transmission power levels of mobile stations (sometimes referred to as mobile terminals or user equipments (UEs)) as low as possible using transmit power control. The power control may be based on some parameter measured or calculated from a received transmission, such as the received power, the signal-to-noise ratio, the signal-to-interference ratio, or other quality parameter. The capacity of the CDMA system is optimal if the base station receives signals from all mobile stations at the same power level. But achieving both accurate and fast power control is difficult in practice, particularly if interference cancellation is used. Power control and interference cancellation affect each other, and therefore, should be considered together. If a transmit power control procedure does not take into account the interference cancellation used by a receiver which improves the quality of the signal in the receiver, then the power control may wrongly set mobile transmit power levels higher than they should be, unnecessarily reducing system capacity. See commonly-assigned US 2002/0021682 A2.
A specific example of this problem can be found in the context of 3G cellular systems. In the 3GPP release 99, the radio access network (RAN) controls resources and UE mobility. Resource control includes admission control, congestion control, channel switching (roughly changing the data rate of a connection). A dedicated connection is carried over a dedicated channel DCH, which is realized as a DPCCH (Dedicated Physical Control Channel) and a DPDCH (Dedicated Physical Data Channel). The inner loop transmit power control tries to maintain a DPCCH signal-to-interference (DPCCH_SIR) level equal to a DPCCH_SIR target regularly updated by outer loop power control, which ensures that the DPDCH is operating at the correct power level (by monitoring the transport block error statistics). The DPDCH power is given by a power offset relative the DPCCH power level. These power offsets are configured during setup of the DCH radio bearer, either by signaling a power offset per transport format or by signaling the power offset of one transport format, and the other power offsets are obtained in the UE via an interpolation/extrapolation procedure. The power offsets may be updated using reconfiguration procedures in the radio access network.
Evolving 3G standards decentralize decision making, and in particular, the control over the short term data rate of the user connection. The uplink data is then allocated to an enhanced dedicated channel (E-DCH), which is realized as an E-DPCCH for data control and an E-DPDCH for data. They are used only when there is uplink data to send. An uplink scheduler (which is located in the base station or the Node B in the evolved standard) determines which transport formats each UE can use over the E-DPDCH. Inner loop, fast power control operates as for normal DCH's, while a slower, outer loop power control adjusts the DPCCH_SIR target to ensure that the E-DPDCH is operating at the correct power level (by monitoring a number of retransmissions in the receiver). Similar to the DCH case, the E-DPCCH and the E-DPDCH power levels are given by power offsets relative the DPCCH power level. These power offsets are configured during setup of the radio bearer either by signaling a power offset per transport format, or by signaling the power offset of one transport format, and the other power offsets are obtained in the UE via an interpolation/extrapolation procedure. Again, the power offsets may be updated using reconfiguration procedures in the radio access network.
Enhanced uplink transmissions from UEs including higher order modulation motivates use of advanced receivers in base stations to more fully exploit the transport formats allowing the highest data rates. Advance receivers will likely use some form of interference cancellation (IC) as mentioned above. One popular form of interference cancellation is successive interference cancellation (SIC) where interference is canceled successively by considering one user's received signal at the time, gradually cancelling more and more interference with the increasing number of considered user signals. In one-stage interference cancellation, one user's received signals are considered, detected, regenerated, and subtracted before detecting and decoding the remaining user signals. The first users processed typically include high data rate users which significantly contribute to the total received signal power. Decoding the user signals after the one-stage interference cancellation is easier and more accurate because of the subtracted interference. Another form of interference cancellation is parallel interference cancellation (PIC) where some or all UE signals are detected in parallel, and corresponding signals are regenerated in an iterative procedure, optimally by considering the channels, correlations, and data of all users and then solving an optimization problem. In response to the reduced interference, lower block error rates or fewer retransmissions are usually detected, and in response thereto, the DPCCH_SIR target level used in outer loop power control is lowered.
Outer loop power control (OLPC) can be slow to converge when changing from one DPCCH_SIR target level to another, which means that if the receiver performance changes rapidly over time, then the outer loop power control will have problems following these changes. Consequently, the gains of the advanced receiver might not be fully utilized in terms of reduced load. Furthermore, a potential concern is whether advanced receivers at a base station can be used to achieve higher loads since outer loop power control might react too slowly, which may cause instabilities in the uplink.
When any kind of interference cancellation procedure is employed, the signal produced after cancellation is delayed by the interference cancellation processing time L. As a result, determining the signal-to-interference ratio (SIR) used in inner loop fast power control is also delayed by the time L. Yet, inner loop uplink power control requires a SIR estimate before interference cancellation in order to be able to send a power control command with a minimum delay to reduce power control performance degradation caused as delays increase.
One possibility is to estimate DPCCH SIR before interference cancellation as is done in U.S. Pat. No. 5,898,740. The validity of that estimate can be questioned because it will be subject to more interference, but the additional processing delay L is avoided. Still, since DPCCH and E-DPDCH are subject to different amounts of interference, suitable power offsets depend on the interference cancellation efficiency which may change over time. Furthermore, cells capable of cancelling interference need different power offsets compared to cells which are not capable of cancelling interference. This means that one set of power offsets is not generally applicable. Consequently, power offset procedures in 3G WCDMA systems are based on the false assumption that both the DPCCH and for example the E-DPDCH experience the same interference.